Open drain drivers are well known, and have many applications. For example, open drain drivers are commonly used for driving a low signal from an associated circuit onto a shared bus to a remote external circuit. The advantage of using an open drain driver in such an application is that when the open drain driver is not required to drive the low signal onto the shared bus, the open drain driver is in a high impedance state, thereby isolating its associated circuit from the shared bus. In such cases, where a plurality of circuits are sharing a bus, the rated supply voltages of the circuits may vary depending on the type of circuit. Some circuits may be operating with a rated supply voltage as low as one volt, while others may be operating with a rated supply voltage of five volts or greater. Accordingly, a high voltage applied to the shared bus by a circuit operating with a high rated supply voltage may be sufficient to exceed the reliability limit of an open drain driver of a lower supply voltage rated circuit sharing the bus. This, in turn, would lead to failure of such an open drain driver.
Open drain drivers are also used for operating transistor switches where the gate of the transistor switch is passively held in one of a logic high state and a logic low state in order to operate the transistor switch in the corresponding one of the conducting and non-conducting states, and the other of the logic high and logic low states for operating the transistor in the other of the conducting and non-conducting states is to be derived from a different source, and in particular, where it is desired that when the transistor switch is operating in response to the gate of the transistor being passively held the one of the logic high and logic low states, the source which provides the said other of the logic high and logic low states presents the gate of the transistor switch with a high impedance, so that the gate of the transistor switch is effectively isolated from the source from which the said other of the said logic high and logic low state is derived. In such cases, an open drain driver is commonly used to apply the alternative logic state to the gate of the transistor switch to that of the passively applied logic state. The advantage of applying one of the logic states to the gate of a transistor switch through an open drain driver is that when the open drain driver is not applying the logic state to the gate of the transistor switch, the open drain driver presents a high impedance to the gate of the transistor switch. Accordingly, if the open drain driver is used to apply the logic state for operating a transistor switch in the non-conducting state, the open drain driver has little or no effect on the signal being switched through the transistor switch when the transistor switch is operating in the conducting state, since the open drain driver presents a high impedance to the gate of the transistor switch when the transistor switch is in the conducting state.
Open drain drivers typically comprise a single field effect transistor (FET) through which the logic state to be applied to the gate of the transistor switch is coupled. A control signal applied to the gate of the single FET selectively operates the FET in the conducting and non-conducting states, so that when in the conducting state, the FET applies the logic state to the gate of the transistor switch, and when in the non-conducting state the FET becomes a high impedance, which thus presents the gate of the transistor switch with a high impedance.
Open drain drivers which comprise a single FET suffer from a serious disadvantage when operating in low supply voltage rated environments, in particular, in CMOS environments, which are designed to operate at rated supply voltages of the order of 5 volts, and in many cases at rated supply voltages of the order of 3.3 volts, and in some cases, at rated supply voltages as low as 1 volt. In such cases the single FET of an open drain driver when operating in the high impedance state cannot tolerate voltages which exceed the rated supply voltage by an amount equal to approximately 10% of the supply voltage. Accordingly, if a shared bus to which an open drain driver is coupled is subjected to a voltage greater than the rated supply voltage of the FET of the open drain driver by an amount equal to approximately 10% of the rated supply voltage when the open drain driver is in the high impedance state, the single FET will fail as a result of the over-voltage. Similarly, in a case where an open drain driver is used to apply the one of a logic high or a logic low state to the gate of a transistor switch, if the gate of the transistor switch is subjected to a voltage higher than the rated supply voltage plus 10% of the single FET of the open drain driver, the single FET will fail as a result of the over-voltage.
In CMOS applications where a MOSFET switch is switching an AC signal, linearity problems arise in the MOSFET switch. The on-resistance of a MOSFET switch varies with the voltage difference across the gate and source or the gate and drain of the MOSFET switch. Thus, as the voltage of the AC signal on the drain-source of the MOSFET switch varies from peak to peak, the voltage across the gate and drain and the gate and source of the MOSFET switch also varies, thereby varying the on-resistance of the MOSFET switch and in turn compromising the linearity of the AC signal switched through the MOSFET switch.
Circuits which overcome the linearity problem of MOSFET switches when switching an AC signal, are disclosed in U.S. published Patent Application No. 2004/0196089, which was filed on Apr. 3, 2003 in the name of John O'Donnell, et al, who is one of the inventors of the present invention, and entitled “Switching Device”. U.S. Application Specification No. 2004/0196089 discloses a MOSFET switch in which the gate is AC coupled to either the source or the drain for maintaining the voltage difference between the gate and the source or drain constant as the voltage of the AC signal varies from peak to peak. However, if such an AC coupled MOSFET switch were passively held in the conducting state and held in the non-conducting state by a logic signal applied to the gate through an open drain driver, the open drain driver would be subjected to the AC voltage which is AC coupled to the gate of the MOSFET switch, and this could result in an over-voltage being applied to the open drain driver when the open drain driver is in the high impedance state, which in turn would lead to failure of the open drain driver.
An open drain driver is disclosed in U.S. Pat. No. 5,028,819 of Wei, et al, which to some extent overcomes the problem of the drain-source voltage of the MOSFET of a single MOSFET open drain driver exceeding the rated supply voltage. Wei proposes the use of two MOSFETs in series in the open drain driver in order to divide any over-voltages to which the open drain driver is subjected across the two MOSFETs, thereby reducing the likelihood of over-voltage failure. In the open drain driver of Wei the drain of the first MOSFET forms the output of the open drain driver, and the source of the first MOSFET is coupled to the drain of a second MOSFET. The source of the second MOSFET is coupled to a voltage source which is to be applied to the output of the open drain driver, when the open drain driver is in a conducting state. The MOSFETs of Wei are N-channel MOSFETs, and the gate of the first MOSFET is constantly biased to the supply voltage VDD, so that the first MOSFET is normally operating in the conducting state. The gate of the second MOSFET is coupled to a control voltage, which selectively and alternately operates the second MOSFET in the non-conducting and conducting states for in turn operating the open drain driver in the non-conducting high impedance state, and in the conducting state for applying the voltage on the source of the second MOSFET to the output of the open drain driver. In the event of an over-voltage being applied to the output of the open drain driver when the open drain driver is in the high impedance state, the first MOSFET conducts until the voltage on the source of the first MOSFET reaches the difference between the voltage on the gate of the first MOSFET and the threshold voltage of the first MOSFET. At that stage the first MOSFET goes into the non-conducting state, and thus the over-voltage applied to the drain of the first MOSFET is divided between the first and second MOSFETs.
However, a problem with the open drain driver of Wei is that the voltage on a node through which the source of the first MOSFET is coupled to the drain of the second MOSFET can increase to a level which exceeds the permitted reliability voltage limit of the second MOSFET, and in certain cases, the first MOSFET. For example, when the first MOSFET is in a non-conducting state as a result of an over-voltage having been applied to its drain, DC current can leak through the first MOSFET. If the rate of current leakage through the first MOSFET is greater than that through the second MOSFET, the voltage on the node coupling the source of the first MOSFET to the drain of the second MOSFET can rapidly reach the breakdown voltage of the second MOSFET. Additionally, when an AC voltage signal is superimposed on a DC voltage on the output terminal of the open drain driver, or when the output terminal of the open drain driver is subjected to a sudden voltage surge, and the open drain driver is in the high impedance state with the first MOSFET in the conducting state, if during a part of the AC cycle of the AC voltage signal, the combined AC and DC voltages, which will appear on the source of the first MOSFET is such that the gate-source voltage of the first MOSFET becomes less than the threshold voltage of the first MOSFET, the first MOSFET will operate in the non-conducting state during that part of the AC cycle of the AC signal while the gate-source voltage of the first MOSFET remains less than the threshold voltage of the first MOSFET. However, since the AC signal and/or the sudden voltage surge will be capacitively coupled to the node coupling the drain of the second MOSFET to the source of the first MOSFET by the drain-source parasitic capacitance of the first MOSFET while the first MOSFET is in the non-conducting state, the voltage on the node coupling the drain of the second MOSFET to the source of the first MOSFET can rise to a level exceeding the breakdown voltage of the second MOSFET. This thus leads to failure of the second MOSFET.
There is therefore a need for an open drain driver with improved over-voltage protection.
The present invention is directed towards providing an open drain driver with improved over-voltage protection, and the invention is also directed towards providing a transistor switch incorporating such an open drain driver, and the invention is also directed towards providing a method for providing an open drain driver with improved over-voltage protection.